The techniques currently used for relative humidity generation conventionally operate under flow rate control wherein a humidity generator continually mixes together a continuous stream of dry air and a continuous stream of saturated, or wet, air, in a selected proportion controlled in accordance with the flow rates thereof, into a mixing chamber or nozzle to generate an output air stream having a desired relative humidity. The dry air stream has a vapor pressure (of water) (V.sub.p) substantially equal to zero while the wet stream has a vapor pressure substantially equal to the vapor pressure at saturation (V.sub.psat). The two streams which are mixed in the mixing chamber produce a mixed air stream having a vapor pressure somewhere between zero and V.sub.psat depending on the ratio of the amounts mixed as controlled by the flow rates thereof into the mixing chamber If the mixture is kept at the saturation temperature of V.sub.psat, the percent relatively humidity (% RH) of the mixture can be determined in accordance with the following equation: ##EQU1## where fd is the flow rate of dry air and fw is the flow rate of wet air. If the vapor pressure of dry air is zero, then equation two can be rewritten as: ##EQU2## if the dry air stream is totally dry (0% RH) and if the wet air stream is completely saturated (100% RH) and if no water vapor is added or removed from the streams after the flows are metered and the temperature and pressure of the generated % RH is the same as the temperature and the pressure of saturation of the wet air stream.
Systems which have used such continuous mixing of selected proportions of wet and dry air have normally required precise measurement and control of the flow rates of the selected proportions of wet and dry air which must be performed using relatively expensive flow sensors or metering devices. Such humidity generating apparatus have generally been extremely large and bulky, as well as relatively costly and somewhat difficult to use.
It is desirable to be able to generate an air stream for supplying to a test chamber to create a desired % RH of the air in the test chamber at a much lower cost while achieving accuracy equivalent at least to that achieved by currently available humidity generators. Moreover, it is desirable to be able to make such devices in a relatively compact manner so that they are not only less bulky in terms of their spatial characteristics but also more light in weight so that they can be made portable so as to be readily carried by a user from one location to another.